OpenCV find sub images - opencv

Question

I have a scanner which is big enough to scan multiple pictures at once.
Unfortunatelly, all the pictures are stored in one jpg file, separated only by
white borders. Is there any way to automatically find the sub images and store them
in separate files? I was thinking about using OpenCV to get the job done, but
I can't find the right functions. Does anybody know which OpenCV function would work, or if there is any other approach (using linux)?
Thanks,
Konstantin

Answer 1

My quick and dirty solution that worked with my images looks like this. I hope people with similar problem can use this as a starting point on how to use OpenCV.
// g++ `pkg-config --cflags --libs opencv` parse.cp
// include standard OpenCV headers, same as before
#include <cv.h>
#include <highgui.h>
#include <stdio.h>
// all the new API is put into "cv" namespace. Export its content
using namespace cv;
using namespace std;
int main( int argc, char** argv )
{
string imagename = argc > 1 ? argv[1] : "lena.jpg";
// the newer cvLoadImage alternative with MATLAB-style name
Mat imgf = imread("original/"+imagename);
if( !imgf.data ) // check if the image has been loaded properly
return -1;
int border = 1000;
Mat img(imgf.rows+2*border,imgf.cols+2*border,CV_8UC3,Scalar(255,255,255));
for (int i=0; i<imgf.cols; ++i) {
for (int j=0; j<imgf.rows; ++j) {
img.at<Vec3b>(j+border,i+border) = imgf.at<Vec3b>(j,i);
}
}
cout << "created border\n";
Mat mask;
img.copyTo(mask);
Scalar diff(2,2,2);
floodFill(mask, Point(0,0), Scalar(0,0,255), NULL, diff, diff);
cout << "flood filled\n";
imwrite("flood.png",mask);
for (int i=0; i<mask.cols; ++i) {
for (int j=0; j<mask.rows; ++j) {
if(mask.at<Vec3b>(j,i) != Vec3b(0,0,255)) {
mask.at<Vec3b>(j,i) = Vec3b(0,0,0);
} else {
mask.at<Vec3b>(j,i) = Vec3b(255,255,255);
}
}
}
cvtColor( mask, mask, CV_RGB2GRAY );
cout << "mask created\n";
imwrite("binary.png",mask);
Mat sobelX;
Mat sobelY;
Mat sobel;
Sobel(mask,sobelX,CV_16S,1,0);
Sobel(mask,sobelY,CV_16S,0,1);
sobel = abs(sobelX)+abs(sobelY);
for (int i=0; i<mask.cols; ++i) {
for (int j=0; j<mask.rows; ++j) {
mask.at<char>(j,i) = abs(sobelX.at<short>(j,i))+abs(sobelY.at<short>(j,i));
}
}
threshold(mask, mask, 127, 255, THRESH_BINARY);
cout << "sobel done\n";
imwrite("sobel.png",mask);
vector<vector<Point> > contours;
vector<Vec4i> hierarchy;
findContours(mask, contours, hierarchy,
CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE );
imwrite("contours.png",mask);
cout << "contours done\n";
// iterate through all the top-level contours
int idx = 0;
for( ; idx >= 0; idx = hierarchy[idx][0] )
{
RotatedRect box = minAreaRect(contours[idx]);
if(box.size.width > 100 && box.size.height > 100) {
Mat rot = getRotationMatrix2D(box.center,box.angle,1.0);
Mat rotimg;
warpAffine(img,rotimg,rot,Size(img.cols,img.rows));
imwrite("rotimg.png",rotimg);
Mat subimg(box.size.width,box.size.height,CV_8UC3);
getRectSubPix(rotimg,box.size,box.center,subimg);
stringstream name;
name << "subimg_"<< imagename << "_" << idx << ".png";
cout << name.str() << "\n";
imwrite(name.str(),subimg);
}
}
imwrite("img.png",img);
imwrite("mask.png",mask);
cout << "Done\n";
return 0;
}